Electric motor stator winding temperature estimation systems and methods

ABSTRACT

An electric motor system includes an electric motor comprising a stator with windings and a rotor configured to operate at a motor speed; a cooling system comprising coolant configured to cool the rotor and the stator, the coolant having a coolant flow rate and a coolant temperature; an inverter module coupled to the electric motor and configured to provide current to the windings based on inverter control signals; a current regulated torque controller coupled to the inverter module and configured to generate the inverter control signals in response to a derated torque command; and a temperature estimation controller coupled to the current regulated torque controller and configured to generate the derated torque command based on an initial torque command and an estimated stator winding temperature. The temperature estimation controller is configured to estimate the estimated stator winding temperature based on the motor speed and the coolant flow rate.

TECHNICAL FIELD

The present invention generally relates to electric motor systems, andmore particularly relates to methods and systems for estimating thetemperature of stator windings in an electric motor.

BACKGROUND

Hybrid electric vehicles (HEVs) typically include an alternating current(AC) electric motor driven by a direct current (DC) power source, suchas a battery. Stator windings of the electric motor may be coupled to apower inverter module that performs a rapid switching function toconvert the DC power to AC power to drive the electric motor, which inturn drives a drivetrain shaft of the HEV.

The temperature of motor stator windings is an important parameter andmay be used for a variety of purposes. For example, stator windingtemperatures may be an input in various motor control algorithms,particularly algorithms that utilize stator resistance as a controlvariable. Additionally, stator winding temperatures can also be used todetect high motor temperatures to prevent overheating. Conventionally,the temperatures of the stator windings are measured by a temperaturemeasurement sensor, such as a thermistor or thermocouple, installed ormounted on one of the stator windings. However, in some systems, theremay be large temperature gradients between the temperature sensor andthe high temperature areas of the stator winding, which may result inaccuracy issues. More than one sensor may be used, although eachadditional sensor raises issues with placement, cost, reliability,service, and maintenance.

To reduce or even eliminate the need for temperature sensors, sensorlessstator winding temperature estimation techniques have also beendeveloped. These temperature estimation techniques may employ complexmotor thermal models based on machine geometry and thermal andelectrical properties. However, in many cases, information regardingsuch motor geometry or thermal or electrical properties may not bereadily available, and the resulting assumptions may result ininaccuracies. Other sensorless stator winding temperature estimationtechniques have been developed that work well for zero or low speedtemperature estimation (e.g., below 75 rpm); however, these techniquesmay not yield accurate results at higher motor speeds.

Accordingly, it is desirable to provide methods and systems forestimating stator winding temperatures over the entire motor speedoperating range (i.e., low operating speeds and high operating speeds)with improved accuracy. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description and the appended claims, taken inconjunction with the accompanying drawings and the foregoing technicalfield and background.

SUMMARY

In accordance with an exemplary embodiment, an electric motor systemincludes an electric motor comprising a stator with windings and a rotorconfigured to operate at a motor speed; a cooling system comprisingcoolant configured to cool the rotor and the stator, the coolant havinga coolant flow rate and a coolant temperature; an inverter modulecoupled to the electric motor and configured to provide current to thewindings based on inverter control signals; a current regulated torquecontroller coupled to the inverter module and configured to generate theinverter control signals in response to a derated torque command; and atemperature estimation controller coupled to the current regulatedtorque controller and configured to generate the derated torque commandbased on an initial torque command and an estimated stator windingtemperature. The temperature estimation controller is configured toestimate the estimated stator winding temperature based on the motorspeed and the coolant flow rate.

In accordance with an exemplary embodiment, a method is provided forestimating stator winding temperatures in a motor having a stator with aplurality of windings and a rotor configured to operate at a motorspeed. The motor is further configured to be cooled by a coolant at acoolant flow rate. The method includes comparing the motor speed to aspeed threshold; generating estimated total power losses of the electricmotor; calculating combined thermal impedances between the plurality ofwindings and the coolant based on the motor speed and the coolant flowrate; and estimating, when motor speed is greater than the speedthreshold, first estimated stator winding temperatures for each of theplurality of stator windings based on the combined thermal impedancesand the total power losses.

DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIG. 1 is a block diagram of an electric motor system in accordance withan exemplary embodiment;

FIG. 2 is a circuit diagram representation of a thermal impedance modelassociated with the system of FIG. 1 in accordance with an exemplaryembodiment;

FIG. 3 is a block diagram of a high speed temperature estimation moduleof the electric motor system of FIG. 1 in accordance with an exemplaryembodiment;

FIG. 4 is a block diagram of a high speed stator winding temperatureestimator of the high speed temperature estimation module of FIG. 3 inaccordance with an exemplary embodiment;

FIG. 5 is a block diagram of a low speed temperature estimation moduleof the electric motor system of FIG. 1 in accordance with an exemplaryembodiment; and

FIG. 6 is a flowchart of the operation of the system of FIG. 1 inaccordance with an exemplary embodiment.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Any embodiment described herein as exemplary is notnecessarily to be construed as preferred or advantageous over otherembodiments and merely serves as an example, instance, or illustration.All of the embodiments described in this Detailed Description areexemplary embodiments provided to enable persons skilled in the art tomake or use the invention and not to limit the scope of the inventionwhich is defined by the claims. Furthermore, there is no intention to bebound by any expressed or implied theory presented in the precedingtechnical field, background, brief summary or the following detaileddescription.

Exemplary embodiments discussed herein relate to methods and systems forestimating the temperature of stator windings in an electric motor. Thedisclosed methods and systems may be implemented in operatingenvironments where it is necessary to estimate the temperature of statorwindings over low and high speeds, including in a hybrid and electricvehicle power system of a hybrid electric vehicle (HEV). For example,the systems and methods estimate the high speed stator windingtemperatures as a function of motor speed and coolant flow rates for amore accurate estimation relative to conventional techniques.

FIG. 1 illustrates a simplified block diagram of a three-phase electricmotor system 100 architecture that may be implemented in a hybridelectric vehicle (HEV). In one exemplary embodiment, the system 100includes a three-phase AC electric motor 110, a three-phase pulse widthmodulated (PWM) inverter module 120, a DC power source 140, a currentregulated torque controller 150, a coolant temperature sensor 156, acoolant flow rate sensor 158, a rotor position sensor 160, and atemperature estimation controller 170. As described in greater detailbelow, during operation, the system 100 receives a torque command (T*)based on, for example, inputs from a driver. Since elevated temperaturesmay result in undesirable issues for the motor 110 and the temperatureof the motor 110 is based, in part, on the torque, the temperatureestimation controller 170 derates or limits the torque command (T*)based on the estimated temperature of the motor 110 to produce a deratedtorque command (T**). This derated torque command (T**) corresponds tothe acceptable torque output of the motor 110 given the torque command(T*) and the current temperature of the motor 110. The current regulatedtorque controller 150 receives the derated torque command (T**) and, inresponse, controls the inverter module 120 to drive the motor 110. Themotor 110 produces a torque on the drive shaft (not shown) of the HEV. Amore detailed description of the system 100 will now be provided.

The motor 110 generally includes a stator with stator windings 115, 116,117 that, when supplied with alternating current, produce a rotatingmagnetic field that causes a rotor (not shown) to rotate and generatetorque. In the depicted exemplary embodiment, the three stator windings115, 116, 117 define a three-phase motor. In general, the motor 110 maybe a permanent magnet synchronous motor, including an interior permanentmagnet motor; an induction motor; a synchronous reluctance motor; or anyother type of suitable electric motor.

The inverter module 120 drives the operation of the motor 110. Theinverter module 120 generally includes a capacitor 180 and threeinverter sub-modules 122, 123, 124, each corresponding to a switchingdevice respectively coupled to the stator windings 115, 116, 117. Eachswitching device 122, 123, 124 includes two switches (e.g., transistorssuch as Insulated Gate Bipolar Transistors (IGBTs)) that operate in analternating manner with antiparallel diodes (not shown) to appropriatelyswitch an input voltage and provide three-phase energization of thestator windings 115, 116, 117 of the motor 110.

The inverter module 120 is connected between direct current (DC) buslines 135 of the DC power source 140 (e.g., one or more batteries orfuel cells) that supplies a DC input voltage (V_(DC)). As noted above,the switching devices 122, 123, 124 supply alternating current (Ia, Ib,Ic) to drive the three-phases corresponding to the stator windings 115,116, 117 of the motor 110 at varying speeds based on the DC inputvoltage (V_(DC)) and control signals from the current regulated torquecontroller 150. Additional details of the current regulated torquecontroller 150 may be found in U.S. patent application Ser. No.12/568,002, filed Sep. 28, 2009 and assigned to the assignee of thepresent invention, which is incorporated by reference herein in itsentirety.

A cooling system 155 with a coolant, such as motor oil, surrounds andcools the motor 110 during operation. The coolant temperature sensor 156determines the temperature of the coolant and provides a digital signalrepresentation of the coolant temperature (T_(COOLANT)). Additionally,the coolant flow rate sensor 158 determines the flow rates (Q_(COOLANT))of the coolant in the rotor and/or stator. As discussed in greaterdetail below, the coolant temperature (T_(COOLANT)) and the coolant flowrates (Q_(COOLANT)) are provided to the temperature estimationcontroller 170 for use in estimating the temperature of the statorwindings 115, 116, 117. In one exemplary embodiment, the coolant flowrate (Q_(COOLANT)) may be measured directly. In another exemplaryembodiment, the coolant flow rate (Q_(COOLANT)) may be derived as afunction of the flow pressure, coolant and motor temperatures, motortorque, and motor speed. In further exemplary embodiments, the coolantflow rate sensor 158 may for a part of, or otherwise communicate with, atransmission control module and/or a hybrid control processor.

The rotor position sensor 160 is positioned to generate absolute angularposition information and/or angular velocity information that correspondto the mechanical angle (θ_(r)) of the rotor and the angular velocity orspeed (ω_(r)) of the rotor. In one exemplary embodiment, the rotorposition sensor 160 may be implemented as a resolver and aresolver-to-digital converter, but can generally be any type of physicalposition sensor or transducer or virtual software implementationthereof, including a Hall Effect sensor or any other similar sensingdevice or encoder that senses the angular position or angular velocityof the rotor. The rotor position sensor 160 provides the angularposition (θ_(r)) and speed (ω_(r)) to the current regulated torquecontroller 150 and the temperature estimation controller 170.

The temperature estimation controller 170 includes a temperaturedependent torque command derater block 172, a high speed temperatureestimation module 174, a low speed temperature estimation module 176,and a transition module 180. The high speed temperature estimationmodule 174 receives synchronous frame currents (I_(d), I_(q)) from thecurrent regulated torque controller 150 and estimates the phasetemperatures (T_(aH), T_(bH), T_(cH)) of the stator windings 115, 116,117 at high speeds. As discussed in greater detail below, the estimatedtemperatures (T_(aH), T_(bH), T_(cH)) are generated based on thesynchronous frame currents (I_(d), I_(q)), rotor speed (ω_(r)), the DCvoltage (V_(DC)), the coolant temperature (T_(COOLANT)), and the coolantflow rates (Q_(COOLANT)). The low speed temperature estimation module176 receives the detected current values (I_(a), I_(b), I_(c)) andestimates the phase temperatures (T_(aL), T_(bL), T_(cL)) of the statorwindings 115, 116, 117 at low temperatures based on the current values(I_(a), I_(b), I_(c)) and the coolant temperature (T_(COOLANT)).

The estimated phase temperatures (T_(aH), T_(bH), T_(cH)) from the highspeed temperature estimation module 174 and the estimated phasetemperatures (T_(aL), T_(bL), T_(cL)) from the low speed temperatureestimation module 176 are provided to the transition module 180.Depending on the rotor speed (ω_(r)), the transition module 180 providesone set of the estimated phase temperatures (T_(a), T_(b), T_(c)) totemperature dependent torque command derater block 172. In one exemplaryembodiment, high speeds correspond to rotor speeds (ω_(r)) greater than75 rpm, while low speeds correspond to rotor speeds (ω_(r)) less than 75rpm, although the selection of the threshold between high and low speedmay vary.

As noted above, the temperature dependent torque command derater block172 modifies the torque command (T*) in response to the selected set ofphase temperatures (T_(a), T_(b), T_(c)) to generate a temperaturederated torque command (T**). The current regulated torque controller150 controls the operation of the inverter module 120, and thus themotor 110, to produce the output torque based on the derated torquecommand (T**).

Accordingly, the operational control signals apply the gain representedby the temperature derated torque control signal (T**) to the commandsignals applied to the inverter module 120. Thus, the currents at eachof the stator windings 115, 116, 117 are received and modified by thecurrent regulated torque controller 150 in response to the temperaturederated torque control signal (T**) to provide appropriate gain to theoperational control signals while integrating a temperature dependenttorque derating into the control structure at all speeds. Accurateestimation of the temperature of each stator winding 115, 116, 117 mayprevent overheating of the motor 110 while providing efficientoperation.

FIG. 2 is a circuit diagram representation of a thermal impedance model200 in accordance with an exemplary embodiment. The thermal impedancemodel 200 may be used by the high speed temperature estimation module174 in accordance with an exemplary embodiment to determine theestimated winding temperatures (T_(aH), T_(bH), T_(cH)) at high speeds.

The thermal impedance model 200 depicted in FIG. 2 may be expressed asEquation (1), as follows:Temperature Change=Thermal Impedance*Total Power Dissipation  (1)For example, the temperature difference (ΔT_(x)) between the temperature(T_(x)) of the stator winding and coolant temperature (T_(COOLANT)) isequal to the product of the thermal impedance (R_(thx)) and powerdissipation (P_(x)) for a particular phase. The thermal impedance model200 is described more fully below with reference to Equations (4)through (6).

When the rotor speed (ω_(r)) is above a particular value (e.g. 75 rpm),the estimated winding temperatures (T_(aH), T_(bH), T_(cH)) may becalculated based on a thermal impedance (R_(th)) between the statorwindings 115, 116, 117 and coolant. In the depicted diagram, the thermalimpedance (R_(tha)) is the thermal impedance between the temperature(T_(a)) of the first winding 115 and the coolant temperature of themotor coolant (T_(COOLANT)), the thermal impedance (R_(thb)) is thethermal impedance between the temperature (T_(b)) of the second winding116 and the coolant temperature (T_(COOLANT)), and the thermal impedance(R_(tbc)) is the thermal impedance between the temperature (T_(c)) of athird winding 117 and the coolant temperature (T_(COOLANT)).

Power dissipation due to stator winding (or copper) loss and stator core(or iron) loss can be expressed using Equations (2) and (3),respectively, as follows:

$\begin{matrix}{{P_{cu} = {R_{DC}i_{x}^{2}}},{R_{DC} = \frac{N_{c}{Nl}_{turn}}{A_{turn}\sigma_{cu}}}} & (2)\end{matrix}$

where R_(DC) is the DC resistance per phase; i_(x) is the stator currentin a particular phase x, N_(c) is the number of coils in a series; N isthe number of turns per coil; l_(turn) is the length of one turn; andA_(turn) is the area of one turn; and σ_(cu) is the conductivity ofcopper; and

$\begin{matrix}{P_{iron} = {{P_{h} + P_{e}} = {{{ɛ_{h}( \frac{f}{f_{n}} )}B_{m}^{\alpha}} + {{ɛ_{e}( \frac{f}{f_{n}} )}^{2}B^{2}}}}} & (3)\end{matrix}$

where P_(iron) is the core (or iron) power loss; P_(h) is the powerdissipation due to hysteresis losses; P_(e) is the power dissipation dueto eddy current losses; B and B_(m) are the peak flux density; α, ε_(h),and ε_(e) are constants for the particular core material; f is theoperating frequency of the motor; f_(n) and is the fundamental nominalfrequency of the motor.

As shown in Equation (3), at low motor operating speeds, core losses(P_(iron)) are negligible since operating frequency (f) is a function ofrotor speed (ω_(r)). However, at higher operating speeds, the operatingfrequency (f) increases and core losses (P_(iron)) become moresignificant. Accordingly, these core losses (P_(iron)) should beaccounted for at high operating speeds to improve accuracy of theestimation. In general, heat generated in the motor 110 includes heatgenerated due to winding losses (P_(cu)) and core losses (P_(iron)) whenusing the high speed temperature estimation module 174. The heatgenerated by windings losses (P_(cu)) may be calculated using the statorcurrents and stator resistances described above with reference toEquation (2).

The thermal impedance in each phase includes thermal impedance betweenthe stator winding and the stator core and the thermal impedance betweenthe stator core and the motor coolant. For example, in a particularphase, the thermal impedance can be represented as Equation (4), asfollows:R _(thx) =R _(wcx) +R _(ccx)  (4)

where R_(thx) is the thermal impedance between the stator winding andthe motor coolant, R_(wcx) is the thermal impedance between the statorwinding and stator core, and R_(ccx) is the thermal impedance betweenthe stator core and motor coolant.

As such, at high speeds, the estimated temperature of the statorwindings 115, 116, 117 may be estimated using the thermal impedances(R_(tha), R_(thb), R_(thc)) and Equations (5), (6) and (7) as follows:

$\begin{matrix}{T_{a} = {{{R_{tha}( \frac{1 + {T_{za}s}}{1 + {2\xi_{a}T_{wa}s} + ( {T_{wa}s} )^{2}} )}( {{I_{s}^{2}R_{sa}} + P_{core}} )} + T_{coolant}}} & (5) \\{T_{b} = {{{R_{thb}( \frac{1 + {T_{zb}s}}{1 + {2\xi_{b}T_{wb}s} + ( {T_{wb}s} )^{2}} )}( {{I_{s}^{2}R_{sb}} + P_{core}} )} + T_{coolant}}} & (6) \\{T_{c} = {{{R_{thc}( \frac{1 + {T_{zc}s}}{1 + {2\xi_{c}T_{wc}s} + ( {T_{wc}s} )^{2}} )}( {{I_{s}^{2}R_{sc}} + P_{core}} )} + T_{coolant}}} & (7)\end{matrix}$

where T_(zx) is the lead time constant [seconds], T_(wx) is the naturaldamped frequency [seconds], ξ_(x) is the damping factor, I_(s) is theRMS stator current value [Amps] computed based on the synchronousreference frame current signals (I_(qs) ^(e), I_(ds) ^(e)), R_(sx) isthe stator resistance [Ω], P_(core) is the stator core/iron loss[Watts], T_(COOLANT) is the motor coolant temperature [°C.]; and xrepresents a, b, or c. It is noted that at zero speed, the statorcurrents (I_(a), I_(b), I_(c)) may not be the same because there will beinstances in which only two phases are carrying current and the thirdphase has zero current flowing. As such, at low speeds, the actualstator currents may be used to compute stator winding losses. However,for high speed estimation, the stator currents (I_(a), I_(b), I_(c)) inall three phases should be the same. As such, stator winding power lossin each phase can be computed using the RMS value (I_(s)) of the motorcurrents.

The thermal impedance model 200 is represented in Equations (5), (6),and (7) by the combination of stator resistance (R_(thx)) and a 2^(nd)order transfer function model that is used to estimate the windingtemperatures (T_(a), T_(b), T_(c)). The bracketed terms in Equations(5), (6) and (7) represent the total power loss (P_(x)) between thestator winding and the motor coolant due to the thermal impedance ofeach phase. For example, the power loss (P_(x)) takes into account thewinding power loss (I_(s) ²R_(sx)) and the core power loss (P_(core)).The thermal impedances, as well as the coefficients of the 2^(nd) ordertransfer function, may be developed empirically offline from measuredtest data. This typically includes measuring phase currents, thetemperature of each phase winding (e.g., with a thermistor orthermocouple), the coolant temperature (T_(COOLANT)) and the coolantflow rates (Q_(COOLANT)). This thermal model characterization may beperformed using an instrumented motor, and the resulting model may beused for online temperature estimation with the same class of motor thatdoes not have any temperature sensors directly on the stator windings.

Now that a description of the model has been provided, FIG. 3 is a blockdiagram of the high speed temperature estimation module 174 of thesystem 100 of FIG. 1 in accordance with an exemplary embodiment. Asnoted above, the synchronous reference frame current signals (I_(d),I_(q)) are received from the current regulated torque controller 150(FIG. 1) by a stator current square magnitude calculator 330. The statorcurrent square magnitude calculator 330 uses the synchronous referenceframe current signals (I_(d), I_(q)) to compute a squared RMS value(I_(s) ²) of the stator current, which is then provided to the powerloss calculators 332, 334, 336. The power loss calculators 332, 334, 336estimate the stator winding resistances (R_(sa), R_(sb), R_(sc))according to Equations (8), (9), and (10), as follows:R _(sa) =R ₂₅(1+α(T _(a)−25))  (8)R _(sb) =R ₂₅(1+α(T _(b)−25))  (9)R _(sc) =R ₂₅(1+α(T _(c)−25))  10)

where the R_(sa), R_(sb), and R_(sc). are the stator windingresistances; T_(a), T_(b), T_(c) are the estimated stator windingtemperatures; R₂₅ designates the stator winding resistance at ambienttemperature (25° C.); and α represents the temperature coefficient ofresistance (typically 0.0039/° C. for copper winding). On a firstiteration (i.e., when the system switches from low speed stator windingtemperature estimation to high speed stator winding temperatureestimation), the power loss calculators 332, 334, 336 use the estimatedstator winding temperatures (T_(aL), T_(bL), T_(cL)) from the low speedstator phase temperature estimator 325, discussed below, or the coolanttemperature (T_(COOLANT)) to determine the stator winding resistances(R_(sa), R_(sb), R_(sc)). On subsequent iterations, the power losscalculators 332, 334, 336 use the previously estimated high speed statorwinding temperature (T_(aH), T_(bH), T_(cH)) provided via a feedbackloop to determine the stator winding resistances (R_(sa), R_(sb),R_(sc)).

The power loss calculators 332, 334, 336 then multiply the squared RMSvalue (I_(s) ²) of the stator currents by the stator winding resistances(R_(sa), R_(sb), R_(sc)) to produce outputs representing stator windingpower losses (P_(SWLA), P_(SWLB), P_(SWLC)), which are then provided tothe high speed stator winding temperature estimator 348.

In addition to the stator winding power losses (P_(SWLA), P_(SWLB),P_(SWLC)), the input voltage (V_(DC)) of the DC voltage source, thetemperature (T_(COOLANT)) of the coolant, the flow rate (Q_(COOLANT)) ofthe coolant, and the rotor speed (ω_(r)) are provided as inputs to thehigh speed stator winding temperature estimator 348. Then, the highspeed stator winding temperature estimator 348 uses these inputs toestimate the high temperature stator winding temperatures (T_(aH),T_(bH), T_(cH)), as described in greater detail below with reference toFIG. 4.

FIG. 4 illustrates a functional block diagram for describing the highspeed stator winding temperature estimator 348 in accordance with anexemplary embodiment. As described above, the stator winding powerlosses (P_(SWLA), P_(SWLB), P_(SWLC)) for each phase are calculatedbased the squared RMS stator current value (I_(s) ²) and statorresistance value (R_(sa), R_(sb), R_(sc)). The stator winding powerlosses (P_(SWLA), P_(SWLB), P_(SWLC)) for each phase are then added tothe motor core loss (P_(core)), which is a function of the motor speed(ω_(r)), RMS stator winding current (I_(s)), and DC voltage (V_(DC)). Anumber of lookup tables (LUTs) 510-1 . . . 510-n in the high speedstator winding temperature estimator 348 may provide the motor corelosses (P_(core)) at various DC voltages (V_(DC)), motor speeds (ω_(r))and RMS currents (I_(s)). Interpolation (e.g., linear interpolation orother known interpolation techniques) may be used to further refine theresulting core losses (P_(core)) between LUT values.

The combination of the stator winding power losses (P_(SWLA), P_(SWLB),P_(SWLC)) and the core power losses (P_(core)) result in total powerlosses (P_(a), P_(b), P_(c)). For reference, the total power losses(P_(a), P_(b), P_(c)) represent the (I_(s) ²R_(sx)+P_(core)) term ofEquations (5), (6), and (7). The total power losses (P_(a), P_(b),P_(c)) are subsequently provided as inputs to thermal impedance models514.

The thermal impedance model 514 determines the

$R_{tha}( \frac{1 + {T_{za}s}}{1 + {2\xi_{a}T_{wa}s} + ( {T_{wa}s} )^{2}} )$term Equations (5), (6), and (7). In particular, the thermal impedancemodel 514 calculates the appropriate lead time constants (T_(za),T_(zb), T_(zc)), natural damped frequencies (T_(wa), T_(wb), T_(wc)),damping factors (ξ_(a), ξ_(b), ξ_(c)), and thermal impedances (R_(tha),R_(thb), R_(thc)).

In one exemplary embodiment, the natural damped frequencies (T_(wa),T_(wb), T_(wc)), the damping factors (ξ_(a), ξ_(b), ξ_(c)), and thethermal impedances (R_(tha), R_(thb), R_(thc)) are each a function ofthe motor speed (ω_(r)) and/or the flow rates (Q_(COOLANT)) of thecoolant. For example, the natural damped frequencies (T_(wa), T_(wb),T_(wc)) are a function of the flow rates (Q_(COOLANT)) of the coolant.The damping factors (ξ_(a), ξ_(b), ξ_(c)) are a function of the flowrates (Q_(COOLANT)) of the coolant. The thermal impedances (R_(tha),R_(thb), R_(thc)) are a function of the motor speed (ω_(r)) and the flowrates (Q_(COOLANT)) of the coolant. The lead time constants (T_(za),T_(zb), T_(zc)) may be a function of flow rates (Q_(COOLANT)).

Accordingly, a number of lookup tables (LUTs) 512-1 . . . 512-n areprovided to provide the lead time constants (T_(za), T_(zb), T_(zc)),the natural damped frequencies (T_(wa), T_(wb), T_(wc)), the dampingfactors (ξ_(a), ξ_(b), ξ_(c)), and the thermal impedances (R_(tha),R_(thb), R_(thc)) based on the inputs of the motor speed (ω_(r)) and thecoolant flow rates (Q_(COOLANT)). As above, interpolation may be used tofurther refine the resulting LUT values. These values may be developedempirically off-line from measured test data. This generally involvesvarious measurements over a number of motor speed (ω_(r)) and the flowrates (Q_(COOLANT)) in an instrumented motor to provide thermal modelsthat may be used for temperature estimation without winding temperaturesensors.

The thermal impedance models 514 calculate a change in temperature(ΔT_(an), ΔT_(bn), ΔT_(cn)) for each phase. The change in temperature(ΔT_(an), ΔT_(bn), ΔT_(cn)) is then added to the motor coolanttemperature (T_(COOLANT)) to obtain the high speed estimated statorwinding temperature (T_(aH), T_(bH), T_(cH)) for each phase.

FIG. 5 is a block diagram of the low speed temperature estimation module176 of the system 100 of FIG. 1 in accordance with an exemplaryembodiment. As noted above, stator currents (I_(a), I_(b), I_(c)) areprovided as inputs to the low speed temperature estimation module 176,particularly to combiners 302, 304, 306 that generate waveformsequivalent to the AC RMS currents (I_(a) ², I_(b) ², I_(c) ²) for eachof the stator windings 115, 116, 117. These waveforms are provided toblocks 308, 310 and 312, respectively. Blocks 308, 310, 312 respectivelycalculate the stator phase resistance (R_(sa), R_(sb), R_(sc)) for eachphase in response to a feedback low speed estimated temperature (T_(aL),T_(bL), T_(cL)), similar to the description above with reference toEquations (8), (9), and (10), and multiply the stator phase resistance(R_(sa), R_(sb), R_(sc)) with the AC RMS currents (I_(a) ², I_(b) ²,I_(c) ²) from the output of the combiners 302, 304, 306. The resultingproducts are provided to blocks 314, 316, 318 for calculation of thetemperature rise (ΔT_(an), ΔT_(bn), ΔT_(cn)) due to the thermalimpedance (Z_(θ) _(—) _(an), Z_(θ) _(—) _(bn), Z_(θ) _(—) _(cn)).

Outputs of blocks 308, 310, 312 are also provided to block 320 forcalculation of the temperature rise (ΔT_(nc)) due to the thermalimpedance (Z_(θ) _(—) _(nc)) between the thermal neutral and thecoolant. The outputs of blocks 314, 316, 318, 320, as well coolanttemperature (T_(COOLANT)), are provided as inputs to a low speed statorphase temperature estimator 325 for estimation of the low speedtemperature estimates (T_(aL), T_(bL), T_(cL)). As noted above, the lowspeed temperature estimates (T_(aL), T_(bL), T_(cL)) are used by thetransition module 180 to determine the appropriate temperature estimates(T_(a), T_(b), T_(c)). Additional techniques for estimating statortemperature at low speeds are described in United States PatentApplication Publication Number 2009/0189561 A1, filed Jan. 24, 2008 andassigned to the assignee of the present invention, which is incorporatedby reference herein in its entirety.

While FIGS. 1-5 depict the temperature estimation controller 170including identifiable modules and blocks, it will be appreciated thatthese blocks or modules may be implemented as software modules thatexecute on a microprocessor, and therefore operation of the temperatureestimation controller 170 may alternately be represented as steps of amethod, as will now be described with reference to FIG. 6. As such, FIG.6 illustrates a flowchart of a method 600 for the operation of atemperature estimation controller 170 of the system 100 of FIG. 1 inaccordance with an exemplary embodiment. For clarity, FIG. 1 isreferenced in the description below.

Processing begins when the motor 110 is turned on at step 602. Afterprocessing determines that the motor 110 is turned on at step 602, analternating current (AC) root mean square (RMS) current value iscalculated 604. The copper loss of each of the stator windings 115, 116,117 of the motor 110 is then calculated at step 606, and first thermalimpedances for each of the stator windings 115, 116, 117 of the motor110 are calculated at step 608 in response to the copper loss calculatedat step 606.

At step 610, temperature increases in the stator windings 115, 116, 117due to corresponding thermal impedances are determined. At step 612, thetemperature of the coolant is determined, for example, by the coolanttemperature sensor 156. At step 614, the temperature increases due tothe thermal impedance of the thermal neutral with respect to thetemperature of the coolant is determined, and at step 616, low speedstator winding temperatures are estimated for each phase based onresults generated at steps, 610, 612, and 614.

At step 618, processing determines whether the speed of the motor 110 isgreater than a threshold speed (e.g., 75 rpms). When the speed is lessthan the threshold speed, at step 620 the stator winding temperaturesare set equal to the estimated low speed stator temperatures from step616. The torque command is then derated at step 622 to preventoverheating of one or more of the stator windings 115, 116, 117.Processing then returns to step 602.

When the speed is determined to be greater than or equal to thethreshold speed at step 618, processing proceeds to step 624. At step624 through 632 the high speed stator winding temperatures areestimated. In particular, at step 624, stator winding resistances of thestator windings 115, 116, 117 are determined. As noted above, in a firstiteration, the stator winding resistances may be estimated, for example,using the estimated stator winding temperatures from the low speedstator phase temperature estimator. In subsequent iterations, the statorwinding resistances may be estimated, for example, using the previouslyestimated high speed stator winding temperatures. At step 626,processing then determines stator winding power losses based on statorwinding resistances and the RMS stator currents. At step 628, processingthen determines total power loss in each phase based on stator windingpower losses and core power losses. At step 630, processing estimates astator winding temperature for each phase based on total power losses,motor speed, coolant temperature, and coolant flow rate. For reference,one exemplary implementation of steps 628 and 630 are described abovewith reference to FIG. 4. At step 632, the stator winding temperaturesare set equal to the high speed estimated stator winding temperatures.Finally, in step 622, the estimated stator winding temperatures computedat step 632 are provided to the derater block 172 and used to derate thetorque command. The method 600 then loops back to step 602.

Accordingly, exemplary embodiments discussed above provide systems andmethods for estimating the stator winding temperatures without requiringa temperature sensor directly on the stator winding. In particular, theestimation systems and methods accurately estimate stator windingtemperatures over low speed and high speeds. For example, the systemsand methods estimate the high speed stator winding temperatures as afunction of motor speed and coolant flow rates for a more accurateestimation.

The disclosed embodiments may be applied to a permanent magnetsynchronous AC motor (PMSM), such as an Interior Permanent MagnetSynchronous Motor (IPMSM) and a Surface Mount Permanent MagnetSynchronous Motor (SMPMSM). Additionally, although an AC machine can bean AC motor (i.e., apparatus used to convert AC electrical energy powerat its input to produce to mechanical energy or power), an AC machine isnot limited to being an AC motor, but can also encompass generators thatare used to convert mechanical energy or power into electrical AC energyor power. Moreover, although the disclosed embodiments can beimplemented in operating environments such as a hybrid electric vehicle(HEV), it will be appreciated by those skilled in the art that the sameor similar techniques and technologies can be applied in the context ofother systems. In this regard, any of the concepts disclosed here can beapplied generally to vehicles. Examples of such vehicles includeautomobiles such as buses, cars, trucks, sport utility vehicles, vans,vehicles that do not travel on land such as mechanical water vehiclesincluding watercraft, hovercraft, sailcraft, boats and ships, mechanicalunder water vehicles including submarines, mechanical air vehiclesincluding aircraft and spacecraft, mechanical rail vehicles such astrains, trams and trolleys, etc. In addition, the term vehicle is notlimited by any specific propulsion technology such as gasoline or dieselfuel. Rather, vehicles also include hybrid vehicles, battery electricvehicles, hydrogen vehicles, and vehicles which operate using variousother alternative fuels.

It should be observed that the disclosed embodiments reside primarily incombinations of method steps and apparatus components. Those of skillwould further appreciate that the various illustrative logical blocks,modules, circuits, and algorithm steps described in connection with theembodiments disclosed herein may be implemented as electronic hardware,computer software, or combinations of both. The various illustrativelogical blocks, modules, and circuits described in connection with theembodiments disclosed herein may be implemented or performed with ageneral purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. A softwaremodule may reside in RAM memory, flash memory, ROM memory, EPROM memory,EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or anyother form of storage medium known in the art. An exemplary storagemedium is coupled to the processor such the processor can readinformation from, and write information to, the storage medium. In thealternative, the storage medium may be integral to the processor. Theprocessor and the storage medium may reside in an ASIC.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the exemplary embodiment or exemplary embodiments. Itshould be understood that various changes can be made in the functionand arrangement of elements without departing from the scope of theinvention as set forth in the appended claims and the legal equivalentsthereof.

What is claimed is:
 1. An electric motor system, comprising: an electricmotor comprising a stator with windings and a rotor configured tooperate at a motor speed; a cooling system comprising coolant configuredto cool the rotor and the stator, the coolant having a coolant flow rateand a coolant temperature; an inverter module coupled to the electricmotor and configured to provide current to the windings based oninverter control signals; a current regulated torque controller coupledto the inverter module and configured to generate the inverter controlsignals in response to a derated torque command; and a temperatureestimation controller coupled to the current regulated torque controllerand configured to generate the derated torque command based on aninitial torque command and an estimated stator winding temperature, thetemperature estimation controller configured to estimate the estimatedstator winding temperature based on the motor speed and the coolant flowrate.
 2. The electric motor system of claim 1, wherein the temperatureestimation controller is configured to compare the motor speed to athreshold motor speed, the temperature estimation controller furtherconfigured to estimate the stator winding temperature as a high speedstator winding temperature when the motor speed is equal to or greaterthan the threshold motor speed and as a low speed stator windingtemperature when the motor speed is less than the threshold motor speed.3. The electric motor system of claim 2, wherein the temperatureestimation controller, when the motor speed is equal to or greater thanthe threshold motor speed, is configured to determine a stator windingresistance; determine a stator winding power loss based on the statorwinding resistance and a root mean square stator current; determine atotal power loss of the electric motor based on the stator winding powerloss and a core power loss; generate a temperature change based on thetotal power loss, the motor speed and a combined thermal impedance; andestimate the stator winding temperature based on the temperature change,the coolant temperature, and the coolant flow rate.
 4. The electricmotor system of claim 3, wherein the stator further includes a statorcore, and wherein the temperature estimation controller is configured todetermine the combined thermal impedance based on a first thermalimpedance between the stator winding and the stator core and a secondthermal impedance between the stator core and the motor coolant.
 5. Theelectric motor system of claim 3, wherein the temperature estimationcontroller is configured to determine the core power loss as a functionof the motor speed, a stator winding current, and a DC bus voltage. 6.The electric motor system of claim 3, wherein the temperature estimationcontroller comprises a plurality of lookup tables that correlate themotor speed and the coolant flow rate with a natural damped frequency, adamping factor, and a thermal impedance.
 7. The electric motor system ofclaim 6, wherein the temperature estimation controller is configured togenerate the temperature change based on the plurality of lookup tables.8. A method for estimating stator winding temperatures in a motor havinga stator with a plurality of windings and a rotor configured to operateat a motor speed, the motor further configured to be cooled by a coolantat a coolant flow rate, the method comprising the steps of: comparingthe motor speed to a speed threshold; generating estimated total powerlosses of the electric motor; calculating combined thermal impedancesbetween the plurality of windings and the coolant based on the motorspeed and the coolant flow rate; and estimating, when motor speed isgreater than the speed threshold, first estimated stator windingtemperatures for each of the plurality of stator windings based on thecombined thermal impedances and the total power losses.
 9. The method ofclaim 8, wherein generating step includes combining stator winding powerlosses and core power losses to generate the total power losses.
 10. Themethod of claim 9, wherein the generating step further includesdetermining the stator winding power losses based on stator windingresistances and root mean square stator currents.
 11. The method ofclaim 8, wherein the step of determining the stator winding power lossescomprises: determining alternating current root mean square statorcurrents; and determining the stator winding power losses based on thealternating current root mean square stator currents.
 12. The method ofclaim 8, wherein the calculating step includes generating a phasetemperatures changes based on the total power losses, the motor speedand the combined thermal impedances.
 13. The method of claim 8, whereinthe estimating step further includes estimating the stator windingtemperatures based on phase temperature changes and the coolanttemperature.
 14. The method of claim 8, wherein said combined thermalimpedances comprise first thermal impedances between the stator windingsand stator cores, and second thermal impedances between stator cores andthe coolant.
 15. The method of claim 8, wherein the calculating stepincludes determining the combined thermal impedances with look-uptables.
 16. The method of claim 8, wherein the calculating step includesdetermining the combined thermal impedances with look-up tables thatcorrelate the motor speed and coolant flow rate with a natural dampedfrequency, a damping factor, and a thermal impedance.
 17. The method ofwith claim 8, further comprising the step of: derating a torque commandin response to the first estimated stator winding temperatures.
 18. Amethod comprising the steps of: determining a stator winding resistancefor a stator winding of a motor based on a temperature of the statorwinding and a temperature coefficient of resistance of the statorwinding; determining a stator winding power loss based on the statorwinding resistance; determining a total power loss based on the statorwinding power loss and a core power loss; determining a phasetemperature change based on the total power loss, a motor speed, acoolant flow rate, and a combined thermal impedance model; andestimating a stator winding temperature based on the phase temperaturechange and a motor coolant temperature.
 19. The method of claim 18,wherein the step of determining the phase changes includes generatingthe phase temperature change based on the total power loss, the motorspeed and the combined thermal impedance.
 20. The method of claim 18,wherein the estimating step further includes estimating the statorwinding temperature for each of the plurality stator windings based onthe phase temperature changes and the motor coolant temperature.